The present invention relates to an active matrix liquid crystal display device on which a thin film transistor (hereinafter referred to as TFT) is provided as a switching element, and a manufacturing method therefor.
FIGS. 17(a) to 17(d) are cross-sectional views showing the manufacturing process of the TFT array substrate on which a conventional TFT of the TFT liquid crystal display device having a low resistance signal line. In the drawing, reference numeral 1 shows a transparent insulating substrate such as a glass substrate, numeral 2 shows a gate line provided with a gate electrode on a transparent insulating substrate 1, numeral 3 shows a common line, numeral 5 shows a gate insulating film formed on the gate line 2, numeral 6 shows a semiconductor layer formed on the gate electrode 2 through an insulating film 5, numeral 7 shows an ohmic contact layer formed on the semiconductor layer 6, numeral 8 shows a pixel electrode, numerals 10 and 11 show a source electrode and a drain electrode formed on the ohmic contact layer 7, and numeral 12 shows a passivation film.
There is explained a manufacturing method for a TFT array substrate on which a conventional TFT is provided. First, as shown in FIG. 17(a), a single layer film of a metal having small specific resistance such as Al or Al alloy is formed on the surface of a transparent insulating substrate 1, thereafter patterning is made by using a photoresist film (hereinafter referred to as a resist) formed by photolithography to form a gate line 2 and common line 3. As shown in FIG. 17(b), there are continuously formed by plasma enhanced chemical vapor deposition (plasma CVD) a silicon nitride film which is to become a gate insulating film 5, and amorphous silicon film, impurity-doped n.sup.+ type amorphous silicon film. Thereafter, using a resist formed by photolithography, amorphous silicon film and n.sup.+ type amorphous silicon film are simultaneously patterned, and then on the position above the gate line 2, a semiconductor layer 6 and an ohmic contact layer 7 are formed.
As shown in FIG. 17(c), as a transparent conductive film there is formed an indium tin oxide (ITO) film, thereafter patterning is performed by using a resist by photolithography to form a pixel electrode 8. Next, on the ohmic contact layer 7, a source electrode 10 and a drain electrode 11 are formed. Here, the source electrode 10 and a drain electrode 11 are formed as two layers structure of film having as a lower layer a film of chromium (Cr) or titanium (Ti) as a barrier metal for improving the ohmic contact characteristics with n.sup.+ type amorphous silicon film, and as an upper layer a film comprising aluminum (Al) or Al alloy having small specific resistance due to low resistance. Alternatively, there may be cases where tungsten or the like is added as an Al alloy in order to prevent the Al film and indium tin oxide (ITO) film constituting a pixel electrode 8 which is to become an under layer from galvanic corrosion in the developing solution. As shown in FIG. 17(d), a silicon nitride is formed into a film to form a passivation film 12.
As described above, in order to obtain a liquid crystal display device of low power consumption, in case a signal line of low resistance is constituted of a single layer film made of Al film or Al alloy film, or a multilayered film including an Al type metal film on the surface layer, because the Al type metal is soft, when it is strongly cleaned with a brush or the like, scratches are formed on the film surface. For this reason, in forming a resist for patterning the Al type metal film, cleaning may be performed by jetting pure water or supersonic cleaning, but no cleaning with a brush can be performed. As a result, sufficient removal of dust on the film surface is not attained, dust and resist which remained around the dust form masks, the Al metal films remaining on those portions are not etched to remain, and short circuit occurs to give lower yield. Also, in case of forming a signal line made of a metal film made of Cr, Ti, tantalum (Ta) or molybdenum (Mo), or alloy thereof other than the Al film containing these metals, or in forming a signal line made of a metal having relatively high hardness such as an alloy film other than Al alloy containing these metals, etch residue of metal film occurs though in a small degree by the dust which has not been completely removed by cleaning with a brush (brush cleaning) or by the dust which deposited after the cleaning, leading to lowering of yield.
The present invention has been made to solve the problems as above, and its object is to provide a method for manufacturing a low power consumption liquid crystal display device at high yield by making high aperture ratio, by preventing lowering yield caused by short circuit between the lines, and by manufacturing the fine pattern lines by forming a signal by line using the material having relatively small specific resistance.